The present invention relates to a method of preparing electrically conductive polymeric material by treatment with a viologen salt and to electrically conductive polymeric articles formed thereby.
In recent years electrically conductive polymers have been widely studied because of their potential important commercial applications. Such polymers include polypyrrole and its derivatives and polyaniline and its derivatives.
The aniline family of polymers is an example of one of the most widely studied electroactive polymers because of the ease of synthesis and the unusual nature of its electrical conductivity which can be varied over a wide range. Polyaniline (PANi) in its insulating base form is readily soluble in N-methylpyrrolidinone and can be cast into films or coated on different substrates. The insulating form can be converted to the conductive form by the following methods:
(1) Treatment with protonic acids
PANi in the emeraldine (50% oxidated) base state when treated with protonic acids shows an increase in electrical conductivity. The resulting conductivity is a strong function of the pH of the equilibrating solution. For example, when aqueous HCl is used at pH greater than 4, the PANi remains as the essentially insulating non-protonated form whereas at pHxcx9c0 (i.e. with 1 M HCl), the conductivity increases by 10 orders of magnitude (J. C. Chiang and A. G. MacDiarmid, Synth. Met. 13, 193 (1986)). In the latter case, the imine units of the PANi are protonated to give a N+/N ratio of about 0.5. In this form of doping, there is no change in the number of electrons in the PANi chains.
The electrical conductivity of the PANi treated with protonic acids depends not only on the pH of the equilibrating solution but also on the oxidation states of the PANi. For example, PANi in the leucoemeraldine (100% reduced) state shows only a small increase in conductivity when treated with protonic acids (J. C. Chiang and A. G. MacDiarmid, Synth. Met. 13, 193 (1986)).
(2) Charge transfer interaction with organic electron acceptors
PANi in the emeraldine base state can undergo charge transfer interaction with organic electron acceptors such as tetrachloro-o-benzoquinone and 2,3-dichloro-5,6-dicyano-p-benzoquinone in acetonitrile (S. H. Khor, K. G. Neoh and E. T. Kang, J. Appl. Polym. Sci. 40, 2015 (1990)). An increase in electrical conductivity is achieved and the magnitude of the conductivity is dependent on the type of organic acceptors and acceptor concentration. The charge transfer interaction of emeraldine base with the organic electron acceptor is analogous to that of protonation by HCl, with the hydrogen atoms in the latter replaced by the partially dehalogenated halobenquinone rings. The maximum conductivity achieved is at least an order of magnitude lower than that achievable in protonic acid doping of emeraldine base.
PANi in the leucoemeraldine state undergoes oxidative doping by organic electron acceptors in acetonitrile solution which involves first the oxidation of the amine nitrogen and also the formation of the imine structure through hydrogen transfer from the amine nitrogen to the acceptor. The so-produced imine nitrogen is then in turn doped by the acceptor, as described above for PANi in the emeraldine state (E. T. Kang, K. G. Neoh, T. C. Tan, S. H. Khor, and K. L. Tan, Macromolecules. 23, 2918 (1990)). The maximum electrical conductivity obtained is also about an order of magnitude lower than that obtained from protonic acid doping of emeraldine.
It would be a significant advance in the art if a method of preparing an electrically conductive polyaniline material could be provided which would function irrespective of the oxidation state of the polyaniline. The polyaniline base material may be a substituted or unsubstituted polyaniline. It would be a further significant advance in the art if a method could be provided which did not involve the use of acid or organic solvents such as acetonitrile.
It is accordingly an object of the present invention to overcome, or at least alleviate, one or more of the difficulties and deficiencies related to the prior art.
Accordingly, in a first aspect of the present invention there is provided an electrically conductive polymeric article including a polymeric material capable of exhibiting electrical conductivity;
the polymeric material being rendered electrically conductive by treatment with a viologen salt.
It has surprisingly been found that the electrically conductive polymeric article according to this aspect of the present invention exhibits a high level of electrical conductivity. For example, electrically conductive polyaniline may be prepared with resistances (Rs) decreasing from approximately 1010 to approximately 106 xcexa9/sq or below, irrespective of the oxidation state of the polyaniline material. Polyaniline material may be in the leucoemeraldine (0% oxidation state) or emeraldine (50% oxidation state) states or in any oxidation state between these two states.
Examples of polymeric materials capable of being rendered electrically conductive by treatment with a viologen include polymers derived from aromatic bases such as aniline and its derivatives and from heterocyclic bases such as pyrrole and its derivatives.
Prepared polymeric materials for inclusion in the polymeric articles of the invention include polyaniline and its derivatives and polypyrrole and its derivatives. Polyaniline and polypyrrole are particularly preferred polymeric materials.
The electrically conductive polymeric material may be formed in any suitable manner and may take any suitable shape. The polymeric may be in the form of a film, e.g. a free standing film, a film coating, e.g. a thin film coating, or a powder.
In a preferred aspect the polymeric material may be deposited or supported on a suitable substrate. The substrate may be a fabric, polymeric matrix film or other construct. A polyethylene substrate, e.g. a low density polyethylene (LDPE) substrate may be used.
The viologen salt utilised to dope the polymeric material may be of any suitable type. Viologen salts such as a dihalide salt of a viologen (1,1xe2x80x2-disubstituted 4,4xe2x80x2-bipyridinium dihalide) have been found to be particularly suitable.
The substituents on the bipyridinium molecule may be selected from alkyl or aryl groups. The alkyl groups may be substituted or unsubstituted C1 to C4 alkyl groups. The alkyl groups may be straight or branched chains. Substituents for the alkyl groups may be selected from a wide range of substituents including halogen, phenyl and substituted phenyl. Examples of preferred alkyl groups include methyl, ethyl, propyl, butyl and phenylmethyl (benzyl). The aryl groups may be substituted or unsubstituted aryl or heteroaryl.
Viologen salts may be provided as free compounds, shown below as 1, or in the form of polymers, shown below as II, where:
R and Rxe2x80x2 are independently chosen from alkyl or aryl groups as indicated above;
R2 is an alkylene group, preferably a C2 to C4alkylene group;
Xxe2x88x92 is an anion, preferably a halide ion; and
n is an integer greater than or equal to 2.
Particularly preferred viologens include ethyl viologen dichloride, benzyl viologen dichloride and poly(butyl viologen dibromide). 
In a preferred aspect of the present invention the polymeric material may be directly treated with a viologen to render it electrically conductive.
Accordingly, in a preferred aspect of the present invention there is provided a method of preparing an electrically conductive polymeric article, which method includes providing
a polymeric material capable of exhibiting electrical conductivity; and
a viologen;
contacting a surface of the polymeric material with the viologen for a time sufficient to permit the polymeric material to be rendered electrically conductive.
Although not wishing to be bound by theory it is believed that when the polymeric material is treated with a viologen salt that the polymeric material undergoes oxidative doping and the halide anions are incorporated into the polymer. Surprisingly the method may be conducted at or above room temperature in the presence of air and preferably utilising an aqueous solution of the viologen salt. Thus the use of protonic acids or organic solvents may be avoided.
The rate of the reaction is dependent on the ease of the reduction of the viologen, e.g. a viologen dication, and the transfer of the anions to the polymeric material to form a doped and conductive form of the polymeric material.
Pursuant to the present invention, the reactions may be carried out by contacting the surface of the polymeric material with an aqueous solution of viologen salt. The reactions may continue for a time sufficient to permit a substantial degree of oxidative doping of the polymeric material to be achieved.
The viologen salt may be selected from any suitable materials as discussed above. A viologen dihalide is preferred.
The method of contacting the surfaces of the polymeric material with the viologen may vary widely, depending on the state of the polymeric material and the method selected for the reactions. One simple and preferred method involves immersing the polymeric material in an aqueous solution of the viologen salt, preferably a viologen dihalide, more preferably in excess, with proper agitation. The preferred concentration of the solution is between 0.01 M and 0.5 M.
The method may be conducted at any suitable temperature, e.g. at a temperature between 0xc2x0 and approximately 100xc2x0 C. The method may be conducted in the presence of air and under ordinary room lighting.
The method may be carried out for a period of time which is dependent on the form of the polymeric material (coating, thick film or powder), type of viologen salt and concentration. For example, for a thin PANi coating and a monomeric viologen dichloride at a concentration of 0.1 M, the reaction is rapid and the film turns conductive within 5 mins. For thick PANi films, and using a similar viologen type and concentration, the period of the experiment is extended to 1 day to ensure completion.
After completion of the reaction, the polymeric material may, for example, be blotted dry between filter paper to remove the unreacted viologen salt. The change of the PANi from the insulating base state (brown and blue for emeraldine freestanding film and coating, respectively) to the conductive state (blue and green for freestanding film and coating, respectively) is visually apparent, and may be confirmed by sheet resistance (Rs) measurement, X-ray photoelectron spectroscopy or UV-visible absorption spectroscopy.